Comparative Quantitative Thermography

The comparative quantitative thermography method is an effective method for evaluating the condition of a machine or component by comparing approximate temperature values between identical items, reference values or baselines.

The determination of precise actual temperatures of a component, using infrared thermography in the field, is considered very difficult to obtain. This is due to a certain extent to the physics of infrared thermography, which must take into consideration the multiple parameters that enable a true absolute temperature measurement.

These considerations are emissivity, reflectivity and transmissivity. As a result, estimates of these considerations can be readily made to obtain a component’s approximate temperature, which in most cases is more than sufficient to determine the severity of an adverse condition.

Since it is not always practical to determine the exact temperature or even emissivities of each machine component, the alternative use of comparative thermography becomes more practical. Comparative measurement, unlike qualitative measurement, identifies a thermal deficiency by comparing the temperatures obtained using a consistent emissivity value.

The temperature differential between two or more identical or similar components is measured numerically. Assuming that the environmental conditions for both components are similar, the differential temperature for the given piece of equipment is recorded as being the amount above the normal operating temperature of the similar equipment.

An example of comparative quantitative thermography would be that, if two or more electric motors (see images below) are operating in the same environment and under the same load conditions, and one is experiencing an elevated temperature, this is usually an indication that a deteriorating condition may exist. However, the determination of the temperature difference would then assist in establishing the severity of the condition.

For example, a 5°C differential would be considered minor, whereas a 50°C differential may be considered to be critical. Also, knowing the approximate value of the elevated temperature would provide an indication that the temperature limit of a component may be approaching alarm values.

Therefore, while qualitative measurements can also detect deficiencies, it is the quantitative measurements that have the capability of determining severity.

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